Hoary Alyssum is and introduced erect annual to short-lived perennial forb, growing on stiff leafy stems from 10 to 28 inches high. Stems are hairy and have branching in the upper part.
The leaves are alternate and covered with a grayish down. There is a clump of basal leaves which may or may not be present at flowering time, that are narrowly elliptical, tapering at the base to a stalk and with a more rounded tip. Stem leaves are more linear with pointed to rounded tips and a base that abuts the stem but not clasping. Stem leaves decrease in size toward the top of the stem.
The inflorescence is a rounded cluster of stalked flowers atop the stem. These clusters elongate considerably into a spike as the plant produces fruit and additional flowers continue to open in the upper part of the spike.
The flowers are 4-parted with a yellowish-green hairy calyx that has 4 sepals with long pointed teeth. The 4 white petals are deeply divided at the apex creating a 'mouse-ear' appearance. Flower stalks are hairy. There are six stamens, four in the center being longer than a shorter pair at the side. The shorter ones have a pair of nectar glands at their base. The pistil is a light green with a rounded stigma.
Fruit: Mature flowers produce a hairy, plumb flattened elliptical seed pod with the style persistent at the tip. The seeds are flattened, rounded, with a narrow wing on the margin.
Invasive: See notes at bottom of page.
Habitat: Hoary Alyssum adapts readily to waste places, disturbed soils along paths, roadsides, railroads, etc where the soil is poor, dry to mesic and sunny - or in gardens in fertile soil where there is a free spot. It grows from a taproot. Seeds germinating in the fall produce a rosette of basal leaves that sends up the flower stalk the following year. Early seeds can produce flowering plants the same season. The quantity of germinating seeds crowd out other plants.
Names: The genus Berteroa, is an honorary for the Italian physician Carlo Guisepppe Bertero (1789-1831) who botanized in the West Indies. The species incana is from the Latin for 'hoary' or 'gray' referring to the general appearance caused by the down on the leaves. The author names for the plant classification are: First to classify was - '(L.)' which refers to Carl Linnaeus (1707-1778), Swedish botanist and the developer of the binomial nomenclature of modern taxonomy. His work was amended by ‘DC’ who is Augustin Pyramus de Candolle (1778-1841), Swiss botanist, who influenced Charles Darwin. He studied plants, began a systematic catalogue and has 2 genera named for him.
Comparisons: Although the "mouse-ear" style flower is seen on other plants, when in flower, with its hairy leaves and stem, it is unlikely to be confused with other plants in Minnesota. Elsewhere, there are two other mustard family plants that look similar except that in flower, the petals are yellow. These are Aurinia saxatilis, Basket of Gold; and Alyssum simplex, Wild Alyssum. Diagram below.
Above: The inflorescence is a terminal branched cluster that enlarges as flowers open and mature to seed. Drawing ©Flora of North America
Below: 2nd photo - The 4 white petals are deeply cleft. Two of the six stamens are shorter and have the nectar glands at their base. The pistil stigma is rounded.
Below: 1st photo - Most stems branch in the upper part, each forming a flower cluster. 2nd photo - The cluster elongates as flowers open at the top and seed capsules form below. 3rd photo - The seed pods are plumb and hairy, like the stem, and have the style persistent at the tip.
Below: 1st photo - The linear stem leaves are covered with a fine down and abut the stem but do not clasp. 2nd photo - Hoary Alyssum forms large colonies - here competing with the even more invasive Crown Vetch
Below: Both sides of the leaf have dense fine hair - 1st photo - upper surface, 2nd photo - under side.
Below: Examples of an upper stem leaf, a mid-stem leaf and a basal leaf.
Below: Comparison drawing of plants that look similar to Hoary Alyssum. Drawing ©Flora of North America
Notes: Introduced from Eurasia, Hoary Alyssum has established itself in the temperate belt of North America, absent only from the southern states of the U.S. and the northern Canadian Provinces. In Minnesota it is found throughout the state except for a number of counties in the SW quadrant. Being an annual with high seed production, the plant can appear, disappear and reappear in the Garden.
Invasive: Hoary Alyssum is listed as a noxious weed in 46 states. Minnesota also lists it on the "invasive non-native terrestrial plant list." In prairie areas it is controlled by periodic burning, otherwise pulling small areas of plants or using glyphosate on larger areas will provide control. There is no biological control. The plant is low in crude protein for browsing animals and its ability to grow under dry conditions while continuously flowering allows it to out-compete native plants, reduce diversity and reduce pollinator count.
References: Plant characteristics are generally from sources 1A, 32, W2, W3, W7 & W8 plus others as specifically applied. Distribution principally from W1, W2 and 28C. Planting history generally from 1, 4 & 4a. Other sources by specific reference. See Reference List for details.
Friends of the Wild Flower Garden, Inc. Text and photos are by G. D. Bebeau unless otherwise credited. "www.friendsofthewildflowergarden.org"